In this paper we report measurements of 1/f noise in a crystalline metallic
oxide with perovskite structure down to 4.2K. The results show existence of
localized excitations with average activation energy $\approx$ 70-80 meV which
produce peak in the noise at T $\approx$ 35-40K. In addition, it shows clear
evidence of tunnelling type two-level-systems (as in glasses) which show up in
noise measurements below 30K.Comment: 11 pages, 4 figures, to appear in Phys Rev B, vol 58, 1st Dec issu

A. K. Raychaudhuri

Arindam Ghosh

H. E. Zimmer

J. E. Vancleve

J. H. Scofield

K. P. Rajeev

M. Cherry

M. Rajeswari

M. S. Islam

M. Sagoi

N. Gayathri

N. Hooge

P. Dutta

R. Kree

R. Sreekala

S. Hunklinger

S. Sahling

T. Venkatesan

V. Narayanamurthy

X. Liu

X. Xu

English

arXiv

In this paper we report measurements of 1/f noise in a crystalline metallic oxide with perovskite structure down to 4.2 K. The results show existence of localized excitations with average activation energy approximate to 70-80 meV which produce a peak in the noise at T approximate to 35-40 K. Ln addition, it shows clear evidence of tunnelling-type two-level systems las in glasses) which show up in noise measurements below 30 K

Ghosh, Arindam

Raychaudhuri, AK

Sreekala, R

Rajeswari, M

Venkatesan, T

Name not available

Low-energy excitations in crystalline perovskite oxides: Evidence from noise experiments

In this paper we report measurements of 1/f noise in a crystalline metallic oxide with perovskite structure down to 4.2 K. The results show existence of localized excitations with average activation energy ≈70-80meV which produce a peak in the noise at T≈35-40K. In addition, it shows clear evidence of tunnelling-type two-level systems (as in glasses) which show up in noise measurements below 30 K

Raychaudhuri, A. K.

Sreekala, R.

Rajeswari, M.

Venkatesan, T.

Publications of the IAS Fellows

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Low energy excitations in crystalline perovskite oxides: Evidence
from noise experiments
Arindam Ghosh∗ and A. K. Raychaudhuri†
Department of Physics
Indian Institute of Science, Bangalore 560 012, India
R. Sreekala, M.Rajeswari and T. Venkatesan
Center for superconductivity research
University of Maryland, College Park, MD 20742 ,USA
Abstract
In this paper we report measurements of 1/f noise in a crystalline metallic
oxide with perovskite structure down to 4.2K. The results show existence
of localized excitations with average activation energy ≈ 70-80 meV which
produce peak in the noise at T ≈ 35-40K. In addition, it shows clear evidence
of tunnelling type two-level-systems (as in glasses) which show up in noise
measurements below 30K.
∗email: aghosh@physics.iisc. ernet.in
†On lien to National Physical Laboratory, New Delhi 110012, India.
1
A number of solids exhibit excitations which are of much lower energy than the charac-
teristic energy scale of typical lattice excitations. Most of the time the low energy excitation
arises from defect motions where atoms or group of atoms have more than one equilibrium
configurations of almost equivalent energy separated by shallow barriers which are accessible
by tunneling or thermal activation. A particular type of such defect motions are atomic or
molecular tunneling of defects seen in a number of doped crystalline systems1,2. In case
of glasses and other amorphous solids the tunneling centers are of a special kind(popularly
called Two Level Systems) which give rise to the Universal low temperature properties of
glasses3. This particular field has been well studied by a number of techniques. Recently,
very sensitive torsional oscillator experiments have been used to study the evolution of low
energy excitations in amorphous silicon4. It has also been known for some time that certain
crystalline solids including metals and metallic alloys show glass-like low energy excitations5.
In this communication we report existence of low energy excitations in three dimensional
cubic perovskite oxides which were observed through sensitive noise experiments.
It has been known since a decade that the cuprate superconductors (belonging to per-
ovskite class) and perovskites with ABO3 structure can posses ”glass-like” low energy ex-
citation originating from oxygen defects6. In recent years, as the quality of the high Tc
crystals and films improved, the observed magnitude of these excitations have become less,
often falling far short of the detectability of the techniques used, like specific heat.
We have undertaken this experiment to investigate the following issues: (i) to establish
that existence of low energy excitations may be generic in ABO3 type perovskite oxides and
(ii) to establish that noise experiments can detect such excitations which cannot be detected
by other techniques. In particular, the existence of low energy excitations in epitaxial films
of perovskite oxide will have implications on the transport properties and is a new addition
to our knowledge of oxygen dynamics of these materials. To our knowledge this is the first
experimental determination of noise in normal conducting perovskite oxides down to low
enough temperature so that the low energy excitations can be detected.
The choice of the material for our study is LaNiO3−δ. This is a perovskite oxide with
2
almost cubic structure7. It is electrically conducting in stoichiometric form (δ ≈ 0). A num-
ber of experiments (including noise experiments) show that the oxygen is highly mobile in
this oxides when δ ≈ 0. Under normal pressure the oxygen diffuses out of the system even at
room temperature and δ increases as observed recently through sensitive noise experiments8.
Electronic transport in this system is strongly linked to oxygen stoichiometry(δ) and
resistivity(ρ) increases sharply as δ increases9. This makes electronic conduction noise a
very sensitive probe of oxygen dynamics in this solid. In a previous study8 we have shown
that noise experiments can detect long range oxygen diffusion for T¿150K. In this paper
we show that there exists clear evidence of low energy excitations in these solids which
most likely arises from localized dynamics of ”frozen defects” of the oxygen lattice at lower
temperatures (T¡100K). It is not necessary however, that these excitations can be called
”glass like”.
Epitaxial films of LaNiO3−δ with nominal thickness ≈ 150 nm were deposited on LaAlO3
substrates by pulsed laser deposition (PLD) using excimer laser. The details of film prepa-
ration is given elsewhere16. The epitaxy of the films are tested by X-ray diffraction. The
resistivity of the film was measured by a four probe technique over the temperature range
4.2K¡T¡380K. The film used by us had ρRT ≈ 0.6 mΩ-cm and ρ4.2K ≈ 0.25 mΩ-cm and an
estimated δ ≈ 0.05. This is close to but somewhat larger than the resistivity obtained in
best bulk samples prepared by high pressure oxygen treatment17.
The noise was measured by a five probe ac technique (carrier frequency 377 Hz) using
samples of bridge type configuration with active volume for noise detection (Ω) ≈ 5× 10−8
cm3 with peak current density ∼ 104 A/cm2. The noise determination used digital signal
processing techniques described elsewhere10. The temperature control during noise determi-
nation was better than 10mK. The background noise primarily consisted of Johnson noise
4kBTR from the sample. Quadratic dependence of the spectral power density Sv(f) on Vbias
ensured ohmic character of the noise.
To normalize our data taken at different frequencies we use a quantity(γ/n) defined as12:
3
γ/n = fΩSv(f)/V
2
bias (1)
where n is the carrier density, Sv(f) is the spectral power density at frequency f measured
with bias Vbias. This is justifiable given the fact that Sv(f) ∝ V
2
bias and it has nearly 1/f
dependence. [In general Sv(f) has 1/f
α dependence with the exponent close to 1] We express
our results as γ/n because there are uncertainties in exact value of the carrier concentration
n.Such uncertainties however do not affect the main conclusions of this work. The quantities
on the right hand side are all experimentally measurable. In fig.1 we show the normalized
power (γ/n) as a function of temperature at three frequencies. A few plots of typical noise
spectra are shown in fig.2a. The spectra are shown as Sv(f) vs. f to accentuate the deviation
of α from 1. Beyond 60K noise increases monotonically with T. The rapid increase of γ/n
above 150K is associated with long range oxygen diffusion. This particular issue has been
discussed previously8. In this brief report we focus the region T¡60K where the long range
diffusion has ceased.
Below 60K there are distinct peaks in noise power as a function of T. The peak positions
as well as the peak heights of both the peaks depend on the measuring frequencies. The
peak positions shift to higher temperatures with increasing frequency which we interpret
as a signature of thermally activated process. From fig.2 we can also see that there is a
perceptible change in α as we cross the peak region.
If a thermally activated process with a characteristic time scale τ governs the underlying
dynamics associated with the distribution of the electron scattering centers, it can give rise
to excess noise in the frequency scale ∼1/τ . Dutta-Horn model envisages such thermally
activated centers as the origin of 1/fα noise provided the activation energy has a broad distri-
bution with respect to kBT
13. As we see from fig.1 for T¡35K the noise magnitude decreases
rather slowly with decreasing temperature which requires a broad (≫kT) distribution of
activation energies at these temperatures. It is this observation that has prompted us to use
the Dutta-Horn theory to extract the density of states(DOS) D(E) in the dynamics of the
scatterers from the observed temperature variation of γ/n. In the framework of the Dutta-
4
Horn model, the energy E is related to τ through the relation τ = τ0 exp[E/kBT], τ
−1
0 being
the attempt frequency. The value of τ0 (≈ 3×10
−11 sec) was obtained from the shift in peak
temperatures for different observation frequencies. We show in fig.3 the result of calculation
of D(E) based on this model. In this model E ≈ -kBTln(ωτ0) where ω = 2pif is the measuring
frequency. We find that the measurements of γ/n for all the three frequencies give D(E) that
peak around 70±4 meV and 84±10 meV. To check the self-consistency of the analysis we
calculated the exponent (α) at 5Hz from the observed spectra at different temperatures and
compared the values with that predicted by the Dutta-Horn model (fig.2b). We used the
relation α = 1 - (∂lnS/∂lnT - 1)/ln(ωτ0). ∂lnS/∂lnT was calculated from a smooth curve
fit to the experimental data. It is cear from the figure that observed α agrees well with
the prediction. Thus the analysis of the spectra in the framework of the Dutta-Horn model
points to the existence of a broad flat spectrum of low energy relaxing states accompanied
by two distinct peaks.
LaNiO3 belongs to ABO3 class of perovskite oxides in which the oxygen is the mobile
species and defect chemistry plays an important role14. The oxygen diffusion in this material
is mediated by defects like oxygen vacancy and has an activation energy ∼ 1eV8. In the
temperature range of our interest the long range diffusion is frozen. Only kinetics that
seems to be allowed is local relaxation involving oxygen and oxygen-vacancy combination.
The important question however is that the observed average activation energy 〈E〉 for this
process is ≈ 70 meV which is more than an order of magnitude less than that seen for long
range diffusion. Typical ΘD of this oxide found from specific heat as well as Bloch-Gruneissen
theory is ≈ 350K - 400K which shows the scale of typical lattice vibration to be smaller than
the activation energy associated with the peaks. Thus the activation energy seen by us has
an energy scale in between the lattice vibration scale and the activation energy associated
with long range diffusion. Also there seems to be a split peak in D(E) with activation energy
differing by approximately 10 meV implying that there are two classes of oxygen defect sites
with slightly differing activation energy. Two such different sites can arise if the structure is
5
slightly distorted from the cubic structure.These small distortions from cubic structure do
exist in these oxides7. Our noise experiments thus clearly show the existence of low energy
localized excitation in this solid. In the following we will investigate the region with E ≤ 60
meV.
We find from the D(E) vs. E curve that while D(E) → 0 for E ≥ 100 meV, there are
substantial number of states left with much lower energy and D(E) → Constant for E ≤ 60
meV. It raises the probability of existence of a glass-like excitations in these crystalline solids
with nearly constant density of states. To investigate this point further we continued the
measurement down to T ≈ 4.2K. We find that below 30K the magnitude of the noise is ∝ T
as seen in the inset of fig.1. At T ¡ 30K, defect relaxation by thermally activated processes
freeze out because the relaxation time τ becomes very large. A likely mechanism that can
give rise to such low energy processes at low temperature is tunnelling of atomic or molecular
species as were found in certain doped alkali halide systems1. It has been shown before that
in presence of random strain fields such two level tunnelling defects can broaden out to give
tunnelling states with a broad energy spectrum2,11. It has been shown theoretically that the
temperature dependent relaxation rates of the tunneling states (arising from interactions
with both electrons and phonons) lead to fluctuations in the number density of these states15.
It is known that the tunneling systems in glasses can interact with the electrons and thus
provide an extra scattering mechanism. The fluctuation in the equillibrium number density
of the tunneling states thus gives rise to a fluctuation in the electron scattering rate leading
to noise in the electrical resistivity. The noise power spectrum of this process measured over
a volume Ω is given as15,
Sv(f)
V 2
=
〈C2〉
n2imp
piDTLS
Ω
kBT
ω
(2)
where DTLS is the constant density of states of the TLS, ω = 2pif, C is a dimesionless coupling
constant ∼ 10−2 and nimp is the concentration of impurities which scatter electrons. This
will give rise to a normalized spectral power
γ
n
=
1
2
〈C2〉kBT
DTLS
n2imp
(3)
6
Our experimental observation for T ¡ 30K (inset of fig 3) show a clear linear dependence
on T as mentioned before and the power spectra are also 1/f type. This we consider as
an evidence for existence of glass-like low energy excitations in these oxides. Taking nimp
as the number density of oxygen vaccancies (our sample has δ ≈ 0.05) we obtain DTLS ∼
1035 erg−1cm−3. This assumes every impurity atom is associated with a TLS. However, in
glasses and low doped alkali halides it is known that the TLS arises from a group of impurity
atoms and nimp ≪ oxygen defect density. In that case DTLS ≪ 10
35 erg−1cm−3. A DTLS ≈
1031-1033 erg−1cm−3 is found in oxide glasses3.
In order to cross-check whether the flatness of the DOS at low energies is a necessary
condition to explain the observed temperature dependence of noise, we carried out calcula-
tions of the noise magnitude as a function of temperature with several trial DOS. Assuming
an activated scattering mechanism we can write13,
S(f, T ) =
∫ ∞
0
D(E)
2τ0e
E/kT
1 + (2pifτ0)2e2E/kT
dE (4)
We find that usual bell shaped DOS with width ≫ kT does not produce the observed
temperature dependence of noise particularly at lower temperatures. Best fit to the data
was obtained with DOS that have a flat distribution D(E) ≈ constant for E ¡ 70 meV. In
fig.4 we have shown the calculation based on two typical choices of DOS as examples. The
dotted line is calculated from a Lorenzian D(E) having width of ≈ 15 meV and the solid
line is obtained from a Lorenzian with flat and finite low energy tail (inset of fig.4). The
need for a flat low energy distribution is clearly visible. Thus our analysis shows that if we
satisfy two basic requirements we can reproduce our data. These two requirements are (a)
a collection of TLS with a broad distribution and (b) a flat DOS at low energy. Apart from
these two general aspects there is nothing unique about the choice of the DOS.
In these perovskite oxides the electronic path is formed by the network of transition
metal and oxygen where the transition metal is at the center of the octahedron formed by
six oxygen ions. The electrical conduction is strongly modified by any defect in the oxygen
lattice. The noise is thus a very sensitive probe of oxygen and oxygen-defect dynamics which
7
directly couples to the scattering centers which give rise to electrical resistivity and resulting
1/fα noise. The low energy defects seen through the noise experiments arise from the oxygen
dynamics in the three dimensional transition metal-oxygen-transition metal network. All
ABO3 oxides in particular has such networks. It may thus happen that the low energy
excitations seen in LaNiO3 is generic to all such ABO3 oxides. In high Tc cuprates, however,
there are different types of chain and plain oxygens. It will not therefore be proper to
comment on existence of low energy excitations in cuprates based on measurements on
LaNiO3.
8
REFERENCES
1 V. Narayanamurthy and R.O. Pohl, Rev. Mod. Phys. 42, 201 (1970)
2 Susan K. Watson, Ph.D. thesis, Cornell University (1992)
3 S. Hunklinger and A.K. Raychaudhuri, Prog. in Low Temperature Phys., 9, 265 (1986)
4 X. Liu, B.E. White Jr. R.O. Pohl, E. Iwanizcko, K.M. Jones, A.H. Mahan, B.N. Nelson,
R.S. Crandall, and S. Veprek, Phys. Rev. Lett. 78, 4418 (1997)
5 J.E. Vancleve, A.K. Raychaudhuri and R.O. Pohl, Z. Physik B93, 479 (1994) and refer-
ences cited therein
6 H.E. Zimmer, J. Engert, P. Franch, Z. Henning and E. Hegenbarth, Ferroelect. letts. 6,
33 (1986) and S. Sahling and J. Sievert, Solid State Communications 75, 237 (1990)
7 K.P. Rajeev, G.V. Shivashankar and A.K. Raychaudhuri, Solid State Communications 79,
591 (1991)
8 Arindam Ghosh, A.K. Raychaudhuri, R. Sreekala, M. Rajeswari and T. Venkatesan, J.
Phys. D: Appl. Phys. 30, L75 (1997)
9 N. Gayathri, A.K. Raychaudhuri, X.Q. Xu, J.L. Peng and R.L. Greene, J. Phys.: Con-
densed Matter 10, 1323 (1998)
10 J.H. Scofield, Rev. Sci. Instrum. 58, 985 (1987)
11 The possibility of noise originating from a single fluctuating two level system (TLS) is ruled
out since the noise spectrum at a particular temperature in that case is a Lorenzian15 with
the frequency exponent α varying from 0 to 2. We, however, observed the variation in α
typically within 1.00±0.15 at all temperatures.
12 N. Hooge, T.G.M. Kleinpenning and L.K.J. Vandamme, Rep. Prog. in Phys. 44, 479
(1981)
9
13 P. Dutta and P.M. Horn, Rev. Mod. Phys. 53, 497 (1981)
14 M. Cherry, M.S. Islam and C.R.A. Catlow, J. Solid State Chem. 118, 125 (1995)
M.S. Islam, M. Cherry and C.R.A. Catlow, J. Solid State Chem. 124, 230 (1996)
15 R. Kree, p285 in ”Noise in Physical Systems” (Editor: C.M. Van Vlet, World Scientific,
Singapore, 1987)
16 M. Sagoi, Appl. Phys. Letts. 62, 1833 (1993)
17 X. Xu, J.L. Peng, Z. Li, H.L. Ju and R.L. Green, Phys. Rev. B 48, 1112 (1993)
10
Figure Caption
Fig.1. Temperature dependence of the normalized noise power (γ/n) at low temperature
at three measuring frequencies. The solid lines are guide to the eye. The inset magnifies the
low temperature (T ¡ 35K) tail of the noise magnitude.
Fig.2. (a) Plots of the noise power at four different temperatures. Spectra at different
temperatures are shifted for clarity. (b) Temperature variation of the spectral slope at 5Hz.
The solid line is the prediction of the Dutta-Horn model obtained from the smooth curve fit
to the noise magnitude (see fig.4).
Fig.3. Distribution of activation energy E obtained from the data using Dutta-Horn
model. Value of 〈τ0〉 (=3×10
11 sec) is obtained from the shifting of the noise peak for
different observation frequency. The solid lines are guide to the eye.
Fig.4. Calculated noise at 5Hz from trial DOS. The solid line fit to the data corresponds
to the DOS shown in the inset (solid line). The dotted line is evaluated from a Lorenzian
DOS (for details see text).
11






Low-energy excitations in crystalline perovskite oxides: evidence from noise experiments

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Low-energy excitations in crystalline perovskite oxides: Evidence from noise experiments

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Low energy excitations in crystalline perovskite oxides: Evidence from
  noise experiments

Low energy excitations in crystalline perovskite oxides: Evidence from noise experiments

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